Radio frequency circuits



:Feb. 13, 1945. v .A. J. MADDOCK 2,369,589

RADIO FREQUENCY CIRCUITS Filed NOV. 24, 1942 2A, 3.4 T C v 4 v Patented Feb. is, was

RADIO FREQUENCY omcm'rs Alan Julian Maddock, London, England, a slignor to International Standard Electric Corporation,

New York, N. Y.

Application November 24, 1942, Serial No. 468,717

' In Great Britain December 9, 1941 2 Claims.v ('CL 179-171) This invention relates to thermionic amplifiers, and in particular to radio transmission systems and the like in which amplifiers are connected in parallel to feed a common load such as an antenna.

In certain classes of high frequency transmit-- ters, particularly in high power radio broadcasting installations, two or more separate thermionic valve amplifier systems or transmitters may be operated in parallel, each such amplifier having its own separate high tension supply for the anodes of the valves. A common exciter unit driving the separate amplifiers is used so that all operate at the same frequency and the outputs may be combined to obtain increased power into the antenna system.

One of the undesirable features of such a system is that should the high-tension anode supply to one amplifier fail while the exciter continues to drive the remaining amplifier or amplifiers, whose anode tension supplies are still operative, high frequency currents will be fed through the output coupling circuits to the amplifier whose high tension supply has failed, with the consequence that these currents are rectified by the valves in this amplifier, and are fed back into the inoperative high-tension supply. High negative voltages appear at the anodes of the valves and at the terminals of the said high tension source,

and are applied to the smoothing filter components. This may be detrimental to the components of the amplifier and coupling system, and

to the high tension supply and its filter equipment.

Means for preventing the above-mentioned rectification should be employed, and it is the object of this invention to provide such prevention by interrupting the radio-frequency excitation to all the amplifiers when the high tension supply to any of them fails. Then no radio frequency currents are present in the coupling cir- 'cuitsand the dangerous conditions outlined above are eliminated.

According to the invention, the interruption of the excitation is brought about by arranging a resistance network connected to each source of anode tension for the final amplifiers in such a manner that it any supply fails the negative bias on one or more of the valves inthe exciter unit is increased thus rendering it inoperative as an amplifier, the change of bias being obtained from the change of currents flowing in the resistance network,

Theinvention will be more clearly understood from the following detailed description and by reference to the accompanying drawing, in which Fig. 1 shows a block schematic diagram of a radio transmitting system in which two amplifiers feed an antenna common to both;

Fig. 2 shows a schematic circuit diagram of an arrangement according to the invention; Fig. 3 shows a generalised schematic circuit diagram or part of Fig. 2.

Fig. 1 shows in block schematic form two amplitying systems, comprising intermediate amplifiers 2A and 2B and final amplifiers 3A and 33 coupled in parallel through the medium of coupling circuit A to a load.or antenna 5; the two final amplifiers are respectively fed with anode tension from the separate sources +A and +8. A commonexciter unit I provides the radio frequency excitation for the two amplifying system. As explained above, should for example the source +B tail, then'radio frequency power will be fed from amplifier 3A through the coupling network 4 to the amplifier 3B, and will be there rectified by the valves. Conversely, if -|-A fails then radio frequency ower is supplied to amplifier 3A. The invention aims at cutting oil the excitation from the exciter unitto both amplifiers in the manner to be described in detail below. The scheme is equally applicable to any number of amplifiers connected in parallel, excited by a common ratio frequency source and having independent sources of anode high tenszon.

Referring to Fig. 2, from the positive terminal of each source of high tension, high resistances RIA, RIB '(Fig. 2) are connected to the opposite corners 2, 4 or the resistance network comprised of four resistances RZA, R3A, R23 and R33, while the other corners l, 3 of the network are connected respectively to the common negative terminal of, the high tension sources and to the bias supply G for one of the valves V in the common exciter unit; the cathode of this valve is connected to corner I of the network.

It will be easily understood that if both the high-tension sources are approximately equal and are operating, and if RIA, IRA and R3A are respectively equal to RIB, B2B and R38, substantially equal currents I will fiow into the corners 2 and 4 or the networ and practically no current will flow in the resistances R3A' and MB. The potential of the corner 3 will therefore be the same as that of the corners 2 and 4, and will be equal to the product of the current I and the resistance of R2A or R23. It will also be obvious that the voltage of corner 3 will be positive. This positive voltage is in series with the ii'xed negative bias provided by source G, and if the resistance values are chosen correctly in conjunction with the voltage of G, the voltage at the grid of the valve V may be made the proper value for operation as an amplifier. It now the high tension supply on one side, for example +B, should fail, there will be no current entering at terminal 4, and that entering at terminal 2 will divide flowing partly through RZA and partly by the path R3A, B3B, RIB in parallel with IRA. There will thus be a smaller positive potential difierence existing between points 3 and I and hence the bias on the valve V will become more negative than before; and by proper choice of values, the increased negative bias can be made such as to cause V to become inoperative as an amplifier. Thus no radio frequency excitation is fed to either of the succeeding amplifying chains and no harm can come to the components connected to supply +B. Similar remarks apply to the case when supply A fails.

The same principle can be extended to any number of amplifiers in the manner indicated in Fig. 3, which shows only the resistance network portion of Fig. 2 and its immediate connections.

It is supposed that there are altogether m-l-n amplifiers, all with separate equal anode supplies, and connected at their outputs to a common load. Of these supplies, n are connected ,to corner 2 and m to corner 4 of the network of Fig. 3. Only two of these are shown on each side, denoted +A, +N and +13, +M, respectively. Each is connected to the network through a high resistance which is large compared with any of the bridge resistances, which are 13/11 between corners I and 2, and 2 and 3, and R/m between corners I and 4, and 4 and 3 as shown. A current I is supposed to enter the bridge from each of the above mentioned supplies. When all are operating, the total cur-rent entering at 2 will be M, and the total current entering at 4 will be ml.

It can easily be shown from well known network theory that if the current 11.1 on the A side be supposed to be acting alone, the difference of potential between the corners 3 and I will be nI.R m-i-n and the current divides one part flowing from 2 to I and the other part flowing round the network by the route 2-3-4-I. Similarly, if it be supposed that the current ml on the B side is acting alone, the corresponding difference of potential between the corners 3 and I will be and the current divides, one part flowing from 4 to I and the other .part flowing round the network by the route 4-3-2--I. By the principle of superposition, the difference of potential be-.- tween the corners 3 and I when both currents n1 and 1111 act together will be the sum of the above two expressions, namely R1, the corner 3 being positive to the corner I. It can also be easily shown that the portions of the currents which flow in the arms 2-3 and 3-4 due respectively to the currents n1 and 1221 are equal and opposite, so that the total current when both are actin together is zero in these arms. 11, now, one of the anode supplies fails, for example +A, then the current n1 becomes (n1)I and the correwhich is the case shown in Fig. 2.

accuse nLR m+n in other words, the failure of the supply +A causes a change in the potential of the corner 3 of this corner becoming less positive. It is obvious from symmetry that the same change will be produced by a failure of any one of the supplies on 5i side or any one of those on the B side.

Thus with the arrangements shown in Fig. 3, the potential difference between corners 3 and i of the network will be LR when all supplies are operating, and will be changed by v m-I-n if any one of them fails.

Some particular cases of the arrangement will be given. Suppose there are two amplifiers only, Then m and n are both equal to 1, the four bridge resistances 'are all equal to R and the potential change produced when one supply fails will be IR/Z. For n=2 and m=1 (three amplifiers), RZA and R3A are each R/2, R213 and R33 are each R, and the potential change is IR/3. For n==2 and =2 (four amplifiers) the bridge resistances are all equal to R/2 and the potential change is IR/ l. In all cases, the potential difference when all supplies are operating is the same, namely LR. It will be seen that in order that the network may function, there must be at least one supply connected to each of the corners 2 and 4 of the network; and it is moreover preferable that the number connected on one side should not exceed the number connected to the other side by more than 1. When this condition holds, the failure of one of the supplies causes the maximum change of potential or the corner 3 of the network.

In applying the network to any particular case, it can b seen from Fig. 2 that the grid bias voltage of the valve V is equal to the sum of the positive voltage of the corner 3 of the network and the negative voltage e of the bias source G, that is, it is equal to (cI.R) when all the anode supplies are operating, and to when one of them fails. Thus R and e will be chosen so that the first of these expressions gives a negative bias suitable for the normal operation of the valve and the second expression gives a negative bias suflicient to bias the valve beyond the cut-oft.

- 'Thus, considering the case of two amplifiers, suppose the valve V requires normally a bias of 2 volts negative for correct functioning but that 10 volts negative wouldbring the characteristic beyond the cut-ofi point, and so prevent it from amplifying; then the change of potential I.R/2 required in the resistance network is 8 volts, which would therefore necessitate a value for the bias supply e of 18 volts, since the initial positive opposing voltage would be LR, or 16 volts.

Although in Fig. 3 the network is shown with resistances R111 and R/m in the arms, other relative values are possible if the high resistances r be modified. For example, the our network resistances may be made all equal to R, if the resistances in series with each supply be. made equal to 1m on one side and m.r on the other, and a similar result will be obtained when one of the supplies tails.

While ghe inventtion has been explained in terms of particular examples, and certain numerical valueshave been quoted, it is not intended to be limited to such examples or values, and various modifications in accordance with the principles explained will occur to those skilled in the art. For instance, although in Fig. 2 thepotential obtained from the resistance network is applied to a single valve V 01' the exciter unit, it could also be applied to several of the valves thereof if desired; moreover, the bias change producedby the network could be applied in some other way to the valve, for example to an auxiliary grid instead of to the control grid as shown.

What is claimed is:

1. A high frequency amplifying system comprising a plurality of thermionic valve amplifiers having output and input circuits and each valve containing an anode, a common load to which being connected to at least one valve of the said exciting source; one of the remaining bridge ternegative terminal of each of the said separate sources, the diagonally opposite bridge terminal minals being connected to the positive terminals of at least one of the said high tension sources and the other to the positive terminals 01 the remainder of the said high tension sources, the said remainder comprising at least one; the connection to each of the said positive terminals including a series connected high resistance.

2. A system according to claim 1 in which the said connection to a valve or valves'ot the exciting source includes in series a biassing source of continuous potential arranged to oppose the potential developed in the said connection by the resistance network.

a ALAN JULIAN MADDOCK. 

